How to Calculate Your Power Needs (Worksheet & Guide)

Every time someone asks me “what size power station should I buy?” my first question is always the same: “What are you going to plug into it?” Most people haven’t actually added up their power needs — they guess, and they usually guess wrong. Either they buy too small and run out of juice halfway through an outage, or they buy a massive station they never use to capacity.

This guide gives you a dead-simple worksheet to calculate your exact power needs. No electrical engineering background required. Grab a pen, walk around your house or campsite, and in 15 minutes you’ll know exactly how many watt-hours you need.

Step 1: List Your Devices

Write down every device you plan to power. Be specific — don’t just write “kitchen stuff.” List each individual appliance. Think about two scenarios:

Separating these helps you determine your minimum capacity (must-haves only) and ideal capacity (everything).

Step 2: Find Each Device’s Wattage

Three ways to find wattage:

1. Check the label: Most devices have a label or plate showing wattage (W), voltage (V), and amperage (A). Look on the back, bottom, or near the power cord.
2. Check the manual or manufacturer’s website: Search “[device model] wattage” or check the spec sheet.
3. Measure it: A Kill-A-Watt meter ($20-30) plugs between the device and the wall outlet and shows real-time wattage. This is the most accurate method, especially for devices with variable draw like refrigerators.

If you can only find voltage (V) and amperage (A), multiply them: Watts = Volts × Amps. A device rated 120V / 2A draws 240W.

Important: the wattage on the label is usually the maximum draw. Many devices use less during normal operation. Refrigerators are the biggest example — the label might say 200W, but the compressor only runs 30-50% of the time, so average consumption is 60-100W. A Kill-A-Watt meter over 24 hours gives you the true average.

Step 3: Estimate Hours of Use

For each device, estimate how many hours per day (or per session) you’ll use it. Be realistic:

Step 4: Calculate Watt-Hours Per Device

Wh = Watts × Hours

Simple multiplication. A 60W laptop used for 4 hours = 240Wh. A 10W LED light used for 6 hours = 60Wh.

Step 5: Add It All Up and Apply the Efficiency Factor

Sum all device watt-hours, then divide by 0.85 to account for inverter efficiency losses:

Required Capacity = Total Wh ÷ 0.85

Then add a 20-30% buffer for safety margin:

Recommended Capacity = Required Capacity × 1.25

The Worksheet

Here’s a blank worksheet. Fill in your devices:

Total Wh (Must-haves only): _____ ÷ 0.85 = _____ × 1.25 = _____ Wh (Minimum station size)

Total Wh (All devices): _____ ÷ 0.85 = _____ × 1.25 = _____ Wh (Ideal station size)

Worked Example: Family Home Outage (24 hours)

Must-haves: 2,770 Wh ÷ 0.85 = 3,259 Wh × 1.25 = 4,073 Wh

All devices: 3,800 Wh ÷ 0.85 = 4,471 Wh × 1.25 = 5,588 Wh

This family needs a minimum 4,000Wh station for must-haves, or ideally 5,500Wh+ for everything. With 400W of solar panels providing ~1,500Wh during daylight, a 3,000Wh station could sustain must-haves indefinitely in good weather.

Don’t Forget Startup Surge

Your capacity calculation tells you how big the battery needs to be. But you also need to check that the station’s continuous wattage and surge wattage can handle your devices’ startup demands.

Add up the maximum simultaneous wattage of devices that might run at the same time. If your fridge compressor kicks on (200W running, 1,200W surge) while you’re using the microwave (1,000W), the station needs to handle 1,200W + 1,000W = 2,200W continuous and potentially 2,200W + 1,200W surge = 3,400W surge momentarily.

Rule of thumb: buy a station with continuous wattage at least 1.5x your highest single-device wattage, and surge wattage at least 2x your highest startup surge.

Frequently Asked Questions

Q: My refrigerator label says 200W but I’ve heard it uses less. What number should I use?

Use the average consumption, not the label wattage. A Kill-A-Watt meter over 24 hours gives the most accurate number. Without a meter, estimate 60-100W average for a standard refrigerator (the compressor runs about 30-50% of the time). For the surge/startup calculation, use 3x the label wattage (200W × 3 = 600W surge). Modern Energy Star fridges average 50-80W; older models average 80-150W.

Q: What about devices that cycle on and off, like a CPAP?

CPAP machines draw variable power depending on pressure settings and whether the humidifier is on. Without humidifier: 25-40W average. With heated humidifier: 50-70W average. The best approach is to measure with a Kill-A-Watt meter during a typical night’s sleep. For estimation, use 40W without humidifier and 60W with humidifier, multiplied by your sleep hours.

Q: How do I account for solar panels in my calculation?

Subtract the expected daily solar production from your total daily consumption. Solar production = Panel Watts × Peak Sun Hours × 0.75 (efficiency factor). Example: 200W panel × 5 peak sun hours × 0.75 = 750Wh per day. If your daily consumption is 2,000Wh, your net battery-only need is 1,250Wh. But size your battery for at least one full day without solar (cloudy days happen).

Q: What are “peak sun hours” and how do I find mine?

Peak sun hours represent the equivalent number of hours at full 1,000 W/m² irradiance. It’s not the same as daylight hours — a location with 12 hours of daylight might only have 4-5 peak sun hours because morning, evening, and cloudy periods produce less than full irradiance. US averages: Southwest (AZ, NM) = 6-7 hours; Southeast (FL, TX) = 4-5 hours; Northeast (NY, MA) = 3-4 hours; Pacific Northwest (WA, OR) = 3-4 hours. Search “peak sun hours [your city]” for specific data.

Q: Should I calculate for the worst case or average case?

Calculate for your realistic worst case — the longest outage you’d reasonably expect, or the most power-intensive camping trip you’d take. Then add the 25% buffer. It’s better to have capacity you don’t use than to run out during an emergency. The cost difference between a 1,500Wh and 2,000Wh station is usually $100-200 — cheap insurance for peace of mind.

The Bottom Line

Fifteen minutes with this worksheet saves you from buying the wrong power station. List your devices, find the wattages, multiply by hours, add it up, apply the efficiency factor, and add a buffer. That’s your number. Don’t guess — calculate. Your future self, sitting comfortably during a power outage with a fully functioning refrigerator and charged phone, will appreciate the effort.

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